Aluminum oxide (alumina) is a chemical compound including aluminum and oxygen with the chemical formula Al2O3. Corundum is a crystalline form of alumina, generally with traces of iron, titanium and chromium. Sapphire is a variety of corundum (α-Al2O3). Sapphire substrates are used for several applications including substrates for light emitting diodes (LEDs), transparent windows for infrared (IR) and visible wavelengths used in military imaging applications, and applications that require high strength and good optical transmission.